A local area network (LAN) is a system for directly connecting multiple computers so that they can directly exchange information with each other. LANs are considered local because they are designed to connect computers over a small area, such as an office, a building, or a small campus. LANs are considered systems because they are made up of several components, such as cables, repeaters, switches, routers, network interfaces, nodes (e.g., computers), and communication protocols. Ethernet is one such protocol. Information is communicated through a LAN in frames transported within data packets. (“Frame” and “data packet,” while technically different, are often used interchangeably to describe data carrying the information.)
A LAN switch (or, more generally, a packet switch) is generally defined as a multi-port device that transfers data between its different ports based on the destination addresses and/or other information found in the individual packets it receives. Switches can be used to segment LANs, connect different LANs, or extend the collision diameter of LANs. Switches are of particular importance to Ethernet-based LANs because of their ability to increase network diameter. Additional background information on packet switches can be found in a number of references such as Fast Ethernet (1997) by L. Quinn et al., Computer Networks (3rd Ed. 1996) by A. Tannenbaum, and High-Speed Networking with LAN Switches (1997) by G. Held, all of which are incorporated herein by reference.
Packet switches generally carry three types of traffic: unicast, multicast and broadcast. Unicast traffic consists of packets that travel from a source, or entry, port to a single destination, or exit, port. Multicast traffic consists of packets that travel from one sending port to many destination ports per a destination list within a packet. Broadcast traffic is a special case of multicast traffic wherein the destination list includes all destination ports, and as such issues surrounding multicast apply equally to broadcast traffic.
Multicast traffic poses a problem for packet switches because multicast packets must be replicated within the packet switch. This replication can cause packet switches to fall behind in transmitting frames that follow the multicast frame due to the time required for replication. This replication time is particularly apparent in crossbar switch architectures that require unimpeded access from the sending port to the destination ports to schedule transmission, as the sending port must wait for other traffic that is destined for the destination port to complete. Shared memory switch architectures do not suffer the same fate as all ports have access to the switch memory independent of each other. In shared memory switches, a packet is stored in a central memory and the sending port makes a forwarding decision that notifies the destination ports of the packet's location for transmit. Each destination port can pull a multicast packet from its storage location independent of the other ports. However, in shared memory switches, the response of the destination ports to the forwarding decision can be time-consuming where, for example, it involves a multicast packet. Each of the destination ports on the destination list must request the packet from the shared memory. The time required for this delays further forwarding decisions and can cause congestion in the sending port if additional traffic is received there while the destination ports complete their requests.
Congestion in packet switches can be caused in many ways. In cases where there is more than one port transmitting to a single destination port, congestion at the destination port can occur and the port is said to be oversubscribed. The ratio of the rate that traffic is generated at the sending ports to the rate the destination port can transmit is called the oversubscription ratio. Rate mismatches in the source network media and destination network media can also cause congestion (another case of oversubscription). For example, if traffic travels from port 0 to port 1 on a switch and port 0 runs at 100 megabits per second and port 1 runs at 10 megabits per second, traffic can easily back up waiting to exit port 1. Traffic shaping can also cause congestion. This is a process where the destination port is intentionally curbed back to a lower transmit rate than it is capable for traffic engineering purposes.
Normal packet network traffic does not allow steady state oversubscription. No network switching equipment can buffer infinite data, so oversubscribed conditions on ports are inherently limited by the storage capabilities of the network equipment. However, it is also normal network behavior to have bursts for short periods of time during which network ports will be oversubscribed for any or all of the reasons previously listed. High port count switching equipment creates the opportunity for high oversubscription ratios during these normal traffic bursts.
End stations (such as a node within a LAN) communicate through a packet network by establishing a channel called a session. This session has characteristics that remain constant during the conversation between the end stations. For example, if station A talks to station B through the packet network to transfer a file, when A sends packets, they are all labeled with B's network address as the destination, and A's network address as the source. Other information in the packets sent between A and B will also generally remain constant for a given session—priority, VLAN, network protocol, etc. Each station in a packet network may run multiple sessions with the same or different destination stations. In general, packets within these sessions must arrive in sequence at their destinations—that is, the network equipment must not re-order them.
As a packet from a given session enters a packet switch, the switch must evaluate fields within the packet and make a forwarding decision (where does the packet go?). After making a forwarding decision, packet switches with egress queuing must place multicast packets on more than one transmit queue (a queue being first in/first out storage). As network media speeds increase, the time allowed to perform these queuing operations shrinks. Egress port congestion aggravates this issue. High oversubscription ratios that result from normal packet network operation force egress queuing mechanisms to queue packets from many sources simultaneously to maintain predictable operation. The more work the queuing mechanism performs to handle congestion, the harder it is to perform multicast packet replication.
An objective of this invention, therefore, is to provide a method and means for enhancing the communication of packets, such a multicast packets, through a switching device.